Remove proxygen dependency from fbthrift-python build

Summary:
Remove proxygen as a build dependency for fbthrift-python:
- Delete proxygen-python manifest (no longer needed)
- Remove proxygen-python from fbthrift-python deps
- Remove find_package(proxygen) from CMakeLists.txt
- Skip http2_helper files in symlink_dir_contents (depend on proxygen)
- Remove proxygen include dirs from Cython include path
- Add --enable-shared guard to prevent --disable-shared injection on manifests with explicit --enable-shared

Validated in createdbysk/fbthrift fork: TDD RED/GREEN (347/347 tests pass), CI GREEN.

Reviewed By: vitaut

Differential Revision: D95712264

fbshipit-source-id: 93c634ff2d011366613ace9f062a6bb9de66c493

Author: Satish Kumar

build/fbcode_builder/getdeps/builder.py

@@ -481,7 +481,11 @@ def __init__(
             inst_dir,
         )
         self.args: list[str] = args or []
-        if not build_opts.shared_libs and "--disable-shared" not in self.args:
+        if (
+            not build_opts.shared_libs
+            and "--disable-shared" not in self.args
+            and "--enable-shared" not in self.args
+        ):
             self.args.append("--disable-shared")
         self.conf_env_args: dict[str, list[str]] = conf_env_args or {}