Revert "Support running plugin without installing betterproto"

This reverts commit c88edfd0

betterproto/plugin.py

@@ -1,4 +1,5 @@
 #!/usr/bin/env python
+
 import itertools
 import os.path
 import pathlib
@@ -7,7 +8,6 @@ import sys
 import textwrap
 from typing import List
 
-sys.path.append(str(pathlib.Path(__file__).parent.parent))  # add betterproto to path
 import betterproto
 from betterproto.compile.importing import get_ref_type
 from betterproto.compile.naming import (