Remove avatar markdown

[joshishiv4]

This is PR related to issue #649 wherein I changed the occurence of !avatar syntax from /zulip/integration/git.

Fixes: I changed the occurence of this syntax from 2 files :

1. zulip_git_config.py -> line 41
2. post-receive -> line 50

How did you test this PR?
After successfully removing the !avatar syntax from these locations i ran a commit message locally with dummy entries and found that everything is working fine.

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).
• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

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Recreated from zulip/python-zulip-api#896