Stop php7 image build in OSS benchmarks CI. (#32437)

Follow-up to https://github.com/grpc/grpc/pull/32420.

tools/internal_ci/linux/grpc_e2e_performance_gke.sh

@@ -115,7 +115,6 @@ time ../test-infra/bin/prepare_prebuilt_workers \
     -l "dotnet:${GRPC_DOTNET_GITREF}" \
     -l "go:${GRPC_GO_GITREF}" \
     -l "java:${GRPC_JAVA_GITREF}" \
-    -l "php7:${GRPC_CORE_GITREF}" \
     -l "python:${GRPC_CORE_GITREF}" \
     -l "ruby:${GRPC_CORE_GITREF}" \
     -p "${PREBUILT_IMAGE_PREFIX}" \

tools/internal_ci/linux/grpc_e2e_performance_gke_experiment.sh

@@ -113,7 +113,6 @@ time ../test-infra/bin/prepare_prebuilt_workers \
     -l "dotnet:${GRPC_DOTNET_GITREF}" \
     -l "go:${GRPC_GO_GITREF}" \
     -l "java:${GRPC_JAVA_GITREF}" \
-    -l "php7:${GRPC_CORE_GITREF}" \
     -l "python:${GRPC_CORE_GITREF}" \
     -l "ruby:${GRPC_CORE_GITREF}" \
     -p "${PREBUILT_IMAGE_PREFIX}" \