Remove X (Twitter) from README and checklists (#3018)

Author: yvonnefroehlich

.github/ISSUE_TEMPLATE/release_checklist.md

@@ -48,7 +48,6 @@ assignees: ''
 - [ ] Announce the release on:
   - [ ] GMT [forum](https://forum.generic-mapping-tools.org/c/news/) (do this announcement first! draft on https://hackmd.io/@pygmt. requires moderator status)
   - [ ] [ResearchGate](https://www.researchgate.net) (after forum announcement, add new version as research item via the **code** category, be sure to include the corresponding new Zenodo DOI)
-  - [ ] [Twitter](https://twitter.com/gmt_dev) (after forum announcement)
 ---
 
 - [ ] Party :tada: (don't tick before all other checkboxes are ticked!)

.github/workflows/check-links.yml

@@ -55,7 +55,6 @@ jobs:
           --exclude "^https://doi.org"
           --exclude "^https://www.researchgate.net/project/"
           --exclude "^https://test.pypi.org/simple/"
-          --exclude "^https://twitter.com/"
           --verbose
           "repository/**/*.rst"
           "repository/**/*.md"

README.rst

@@ -46,9 +46,6 @@ PyGMT
 .. image:: https://img.shields.io/badge/Contributor%20Covenant-v2.1%20adopted-ff69b4.svg
     :alt: Contributor Code of Conduct
     :target: CODE_OF_CONDUCT.md
-.. image:: https://img.shields.io/twitter/follow/gmt_dev?style=social
-    :alt: Twitter URL
-    :target: https://twitter.com/gmt_dev
 
 .. placeholder-for-doc-index