cleanup(python): Remove the 'cpp' implementation once support for Protobuf 3.x is dropped.

[parthea]

See the code below which should be removed once support for Protobuf 3.x is dropped.

synthtool/synthtool/gcp/templates/python_mono_repo_library/noxfile.py.j2

Lines 251 to 254 in 6d07042

	# Remove the 'cpp' implementation once support for Protobuf 3.x is dropped.
	# The 'cpp' implementation requires Protobuf<4.
	if protobuf_implementation == "cpp":
	session.install("protobuf<4")

synthtool/synthtool/gcp/templates/python_library/noxfile.py.j2

Lines 246 to 249 in 6d07042

	# Remove the 'cpp' implementation once support for Protobuf 3.x is dropped.
	# The 'cpp' implementation requires Protobuf<4.
	if protobuf_implementation == "cpp":
	session.install("protobuf<4")