Remove code to start Goma compiler_proxy.

Since the code to start compiler proxy is implemented in soong now
(https://android-review.googlesource.com/c/platform/build/soong/+/839293

).
We do not need to do that in goma.mk.

Bug: 118390303
Test: After staring goma client, executed followings:
Test: USE_GOMA=true make -j 4
Change-Id: If3da9908483e183f539c0d6ef9dc5826200b0d6b
Signed-off-by: Yoshisato Yanagisawa <yyanagisawa@google.com>