[NFC] .github: Delete release branch PR template in favor of org-wide variant

.github/pull_request_template.md

@@ -1,12 +1,17 @@
-<!-- What's in this pull request? -->
-Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.
-
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+Otherwise, replace this comment with a description of your changes and
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+If this pull request resolves any GitHub issues, link them like so:
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+  Resolves <link to issue>, resolves <link to another issue>.
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-Resolves #NNNNN, fix apple/llvm-project#MMMMM.
 
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