chore: remove legacy python owlbot pipeline and dependencies (#13912)

This change removes dead pip configurations, CI environment variables,
and the hermetic build Python codebase. All post-processing tasks are
now handled natively in Go by Librarian.

Fixes googleapis/librarian#7064

Author: yangyzs

.github/workflows/librarian_generation_check.yaml

@@ -75,8 +75,6 @@ jobs:
         librarian install
         echo "$HOME/.cache/librarian/bin/java_tools/bin" >> $GITHUB_PATH
     - name: Run librarian generate
-      env:
-        PYTHONPATH: ${{ github.workspace }}/sdk-platform-java/hermetic_build/library_generation/owlbot
       run: |
         librarian generate --all
     - name: Check for generated code changes

.github/workflows/sdk-platform-java-downstream.yaml

@@ -6,7 +6,6 @@ on:
     # Changes to these directories do not directly affect the downstream libraries
     paths-ignore:
       - gapic-generator-java/**
-      - library_generation/**
       - showcase/**
       - test/**
 

.github/workflows/showcase.yaml

@@ -127,13 +127,7 @@ jobs:
       - uses: actions/setup-go@40f1582b2485089dde7abd97c1529aa768e1baff # v5
         with:
           go-version: 'stable'
-      - uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065 # v5
-        with:
-          python-version: "3.12"
-          cache: 'pip'
       - name: Install Librarian and tools
-        env:
-          PYTHONPATH: ${{ github.workspace }}/sdk-platform-java/hermetic_build/library_generation/owlbot
         run: |
           V=$(go run github.com/googleapis/librarian/cmd/librarian@latest config get version)
           echo "Installing librarian version $V"

.github/workflows/update_librarian_googleapis.yaml

@@ -125,7 +125,6 @@ jobs:
         go run "github.com/googleapis/librarian/cmd/librarian@${STEPS_LIBRARIAN_OUTPUTS_VERSION}" install
         echo "$HOME/.cache/librarian/bin/java_tools/bin" >> $GITHUB_PATH
       env:
-        PYTHONPATH: ${{ github.workspace }}/sdk-platform-java/hermetic_build/library_generation/owlbot
         STEPS_LIBRARIAN_OUTPUTS_VERSION: ${{ steps.librarian.outputs.version }}
     - name: Generate Libraries
       if: steps.detect_librarian.outputs.has_changes == 'true'

generation/set_owlbot_config.sh

@@ -39,14 +39,6 @@ tools:
       packaging: jar
       local_path: sdk-platform-java/gapic-generator-java
       main_class: com.google.api.generator.Main
-  pip:
-    - name: PyYAML
-      version: 6.0.2
-    - name: jinja2
-      version: 3.1.6
-    - name: synthtool
-      version: ""
-      local_path: sdk-platform-java/hermetic_build/library_generation
   protoc:
     version: "33.6"
     sha256: 9c49962391ff8b5754342509efc3885db91dfe11a358e1e98c9a71518f0ad199