[ios] Remove obsolete comment about closed bug 1018739

Chromium no longer uses sources assignment filter, so the comment
above the ios_sdk_developer_dir variable no longer applies and it
can be removed.

Bug: 1018739
Change-Id: I6e77678be8a176fe912c51a61536459179f21f04
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2471456
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817083}

build/config/ios/ios_sdk.gni

@@ -22,9 +22,6 @@ declare_args() {
   machine_os_build = ""
 
   # Set DEVELOPER_DIR while running sdk_info.py.
-  # Due to https://crbug.com/1018739, the path must not contains filtered
-  # out patterns or the build will fail. In particular src/build/mac does
-  # not work (see build/BUILDCONFIG.gn for pattern that would cause issue).
   ios_sdk_developer_dir = ""
 
   # Control whether codesiging is enabled (ignored for simulator builds).