Cleanup BUILD.gn files from imports like foo:foo

Repalce all occurrences of foo:foo in deps with just foo in BUILD.gn
files.

Done with Sublime regex replace.
Find: \b([-a-zA-Z0-9_]+):+\1\b
In: *.gn
Replace with: \1

Bug: None
Change-Id: I40aba1b14face687a595b852ffe443cb20197611
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/127899
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27225}

system_wrappers/BUILD.gn

@@ -66,9 +66,9 @@ rtc_static_library("system_wrappers") {
   if (is_win) {
     libs += [ "winmm.lib" ]
 
-    # Windows needs ../rtc_base:rtc_base due to include of
+    # Windows needs ../rtc_base due to include of
     # webrtc/rtc_base/win32.h in source/clock.cc.
-    deps += [ "../rtc_base:rtc_base" ]
+    deps += [ "../rtc_base" ]
   }
 
   deps += [