De-dent protobuf definitions (#529)

The protobuf definitions were indented once. This made it difficult to
read the message definitions as the reader always thinks that the
message definitions are nested.

In this patch, we dedent the message definitions so that they start at
the beginning of the line.

Author: mristin

aas_core_codegen/protobuf/structure/_generate.py

@@ -21,7 +21,6 @@
     Identifier,
     assert_never,
     Stripped,
-    indent_but_first_line,
 )
 from aas_core_codegen.protobuf import (
     common as proto_common,
@@ -527,7 +526,7 @@ def generate(
 package {namespace};
 
 
-{I}{indent_but_first_line(code_blocks_joined, I)}"""
+{code_blocks_joined}"""
         ),
         proto_common.WARNING,
     ]  # type: List[Stripped]