[DevTools] remove /inspector_protocol from .gitignore

This directory is rolled manually and thus far does not
contain anything that's synced in by gclient or similar.
So it's confusing to have it in .gitignore because
it makes it easy to forget updating things in it this way.

an automatic deps mechanism, but for now this should be it.

PS: We may later add this back, if we end up going with
Change-Id: I74441dd50055efa860dd793789786b42c6c0fa2b
Reviewed-on: https://chromium-review.googlesource.com/c/1338720
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608940}

third_party/.gitignore

@@ -107,7 +107,6 @@
 /icu
 /icu4j/lib/
 /intellij/lib/
-/inspector_protocol
 /instrumented_libraries/scripts/*.tgz
 /instrumented_libraries/scripts/out/*
 /javax_inject/lib/