Remove an unused flag in fx_lcms2_warnings config.

The warnings it suppresses no longer occur as of the LCMS 2.8 update in
https://pdfium-review.googlesource.com/10590.

Change-Id: I6bbed3c3d7da238bc15e16170ea12745038e3377
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/69873
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
diff --git a/third_party/BUILD.gn b/third_party/BUILD.gn
index 7d44e2d..4d6f5bf 100644
--- a/third_party/BUILD.gn
+++ b/third_party/BUILD.gn
@@ -263,9 +263,6 @@
       # cmslut.cc is sloppy with aggregate initialization. Version 2.7 of this
       # library doesn't appear to have this problem.
       "-Wno-missing-braces",
-
-      # FindPrev() in cmsplugin.c is unused.
-      "-Wno-unused-function",
     ]
   }
 }