docs: remove dead Gitter badge and link

[ssmurfgg04-gif]

Describe your change:

Removed the dead Gitter badge and the Gitter mention from the README, keeping the Discord link as the community channel.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests?
• Documentation change?

Details: https://gitter.im/TheAlgorithms/community returns 404 (Gitter shut down its community pages). The README currently shows a broken Gitter badge and points people to it under "Community Channels". Discord remains live and is already linked, so the Gitter references are removed.

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one file (README.md). To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory. (n/a: no Python files changed)
• All filenames are in all lowercase characters with no spaces or dashes. (n/a: no new files)
• All functions and variable names follow Python naming conventions. (n/a: no code changes)
• All function parameters and return values are annotated with Python type hints. (n/a: no code changes)
• All functions have doctests that pass the automated testing. (n/a: no code changes)
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation. (n/a: no new algorithms)
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword. (n/a: no open issue for this)

Items not marked are not applicable: this PR only edits README.md (documentation).

[algorithms-keeper]

Closing this pull request as invalid

@ssmurfgg04-gif, this pull request is being closed as none of the checkboxes have been marked. It is important that you go through the checklist and mark the ones relevant to this pull request. Please read the Contributing guidelines.

If you're facing any problem on how to mark a checkbox, please read the following instructions:

• Read a point one at a time and think if it is relevant to the pull request or not.
• If it is, then mark it by putting a x between the square bracket like so: [x]

NOTE: Only [x] is supported so if you have put any other letter or symbol between the brackets, that will be marked as invalid. If that is the case then please open a new pull request with the appropriate changes.