Remove copyright from PR template (#2227)

The template gets inserted into the PR description verbatim, copyright included, which wasn't my intent.

.github/pull_request_template.md

@@ -1,10 +1,4 @@
-<!--
-Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-Exceptions. See /LICENSE for license information.
-SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
--->
-
-_Replace this paragraph with a description of what this PR is changing or
-adding, and why._
+**Replace this paragraph with a description of what this PR is changing or
+adding, and why.**
 
 Closes #ISSUE

.pre-commit-config.yaml

@@ -177,6 +177,7 @@ repos:
         exclude: |
           (?x)^(
               .bazelversion|
+              .github/pull_request_template.md|
               compile_flags.txt|
               third_party/.*|
               bazel/llvm_patches/.*\.patch|