php: remove protobuf-php clone hack

templates/tools/dockerfile/interoptest/grpc_interop_php/Dockerfile.template

@@ -46,11 +46,6 @@
   RUN curl -sS https://getcomposer.org/installer | php
   RUN mv composer.phar /usr/local/bin/composer
   
-  # As an attempt to work around #4212, try to prefetch Protobuf-PHP dependency
-  # into composer cache to prevent "composer install" from cloning on each build.
-  RUN git clone --mirror https://github.com/stanley-cheung/Protobuf-PHP.git ${'\\'}
-    /root/.composer/cache/vcs/git-github.com-stanley-cheung-Protobuf-PHP.git/
-  
   # Download the patched PHP protobuf so that PHP gRPC clients can be generated
   # from proto3 schemas.
   RUN git clone https://github.com/stanley-cheung/Protobuf-PHP.git /var/local/git/protobuf-php

templates/tools/dockerfile/stress_test/grpc_interop_stress_php/Dockerfile.template

@@ -47,11 +47,6 @@
   RUN curl -sS https://getcomposer.org/installer | php
   RUN mv composer.phar /usr/local/bin/composer
   
-  # As an attempt to work around #4212, try to prefetch Protobuf-PHP dependency
-  # into composer cache to prevent "composer install" from cloning on each build.
-  RUN git clone --mirror https://github.com/stanley-cheung/Protobuf-PHP.git ${'\\'}
-    /root/.composer/cache/vcs/git-github.com-stanley-cheung-Protobuf-PHP.git/
-  
   # Download the patched PHP protobuf so that PHP gRPC clients can be generated
   # from proto3 schemas.
   RUN git clone https://github.com/stanley-cheung/Protobuf-PHP.git /var/local/git/protobuf-php

tools/dockerfile/interoptest/grpc_interop_php/Dockerfile

@@ -122,11 +122,6 @@ RUN /bin/bash -l -c "rvm all do gem install ronn rake"
 RUN curl -sS https://getcomposer.org/installer | php
 RUN mv composer.phar /usr/local/bin/composer
 
-# As an attempt to work around #4212, try to prefetch Protobuf-PHP dependency
-# into composer cache to prevent "composer install" from cloning on each build.
-RUN git clone --mirror https://github.com/stanley-cheung/Protobuf-PHP.git \
-  /root/.composer/cache/vcs/git-github.com-stanley-cheung-Protobuf-PHP.git/
-
 # Download the patched PHP protobuf so that PHP gRPC clients can be generated
 # from proto3 schemas.
 RUN git clone https://github.com/stanley-cheung/Protobuf-PHP.git /var/local/git/protobuf-php

tools/dockerfile/stress_test/grpc_interop_stress_php/Dockerfile

@@ -127,11 +127,6 @@ RUN /bin/bash -l -c "rvm all do gem install ronn rake"
 RUN curl -sS https://getcomposer.org/installer | php
 RUN mv composer.phar /usr/local/bin/composer
 
-# As an attempt to work around #4212, try to prefetch Protobuf-PHP dependency
-# into composer cache to prevent "composer install" from cloning on each build.
-RUN git clone --mirror https://github.com/stanley-cheung/Protobuf-PHP.git \
-  /root/.composer/cache/vcs/git-github.com-stanley-cheung-Protobuf-PHP.git/
-
 # Download the patched PHP protobuf so that PHP gRPC clients can be generated
 # from proto3 schemas.
 RUN git clone https://github.com/stanley-cheung/Protobuf-PHP.git /var/local/git/protobuf-php