chore: Remove obsolete files

[alicejli]

We have moved the release jobs to Google3. The requirements.txt
is now unused here.

Internal bug: b/296905003

Note that I had to add entries in owlbot.py because they are templated files.

[generated-files-bot]

Warning: This pull request is touching the following templated files:

• .kokoro/release/bump_snapshot.cfg
• .kokoro/release/common.cfg
• .kokoro/release/common.sh
• .kokoro/release/drop.cfg
• .kokoro/release/drop.sh
• .kokoro/release/promote.cfg
• .kokoro/release/promote.sh
• .kokoro/release/publish_javadoc.cfg
• .kokoro/release/publish_javadoc.sh
• .kokoro/release/publish_javadoc11.cfg
• .kokoro/release/publish_javadoc11.sh
• .kokoro/release/snapshot.cfg
• .kokoro/release/snapshot.sh
• .kokoro/release/stage.cfg
• .kokoro/release/stage.sh