Don't install an unused flex dependency

Summary: AFAICS flex is no longer used (see D36023574 (cba8af9)) after dependency on it was removed from Thrift. Remove it from CI configs.

Reviewed By: Mizuchi

Differential Revision: D36112861

fbshipit-source-id: 7405d7dcab0576889a0dd6ec3152ab9c61a27319

Author: vitaut

.github/workflows/getdeps_linux.yml

@@ -135,8 +135,6 @@ jobs:
       run: python3 build/fbcode_builder/getdeps.py build --no-tests proxygen
     - name: Build bison
       run: python3 build/fbcode_builder/getdeps.py build --no-tests bison
-    - name: Build flex
-      run: python3 build/fbcode_builder/getdeps.py build --no-tests flex
     - name: Build libffi
       run: python3 build/fbcode_builder/getdeps.py build --no-tests libffi
     - name: Build ncurses