Remove testing label from labeler.yml (#1220)

#1219 had an issue with noisy testing label. Investigated if there is a
syntax to make it more exclusive, but turns out there isn't one yet. So
let's remove an manually add it in as needed.

Fixes #1219.

.github/labeler.yml

@@ -9,10 +9,6 @@ documentation:
             - libc/README.md
             - tool/cosmocc/README.md
             - third_party/getopt/README.txt
-testing:
-    - changed-files:
-        - any-glob-to-any-file:
-            - test/**
 build:
     - changed-files:
         - any-glob-to-any-file: