unbundle: remove obsolete GYP script

build/linux/unbundle/README

 This directory contains files that make it possible to use system libraries.
 
 For more info please read the following:
 
  - https://fedoraproject.org/wiki/Packaging:No_Bundled_Libraries
  - https://wiki.gentoo.org/wiki/Why_not_bundle_dependencies
  - http://www.debian.org/doc/debian-policy/ch-source.html#s-embeddedfiles
 
 For more Chromium-specific context please read
 http://spot.livejournal.com/312320.html .
@@ skipping 19 matching lines @@
    Everything that is not explicitly preserved is removed (except for
    gyp files), and the script fails if any directory passed on command
    line does not exist (to ensure list is kept up to date).
 
    This is intended to be used on sources extracted from a tarball,
    not a repository.
 
    NOTE: by default this will not remove anything (for safety). Pass
    --do-remove flag to actually remove files.
 
-2. Pick the script to run depending on whether you use GYP or GN:
+2. replace_gn_files.py --system-libraries lib...
 
-2a. replace_gyp_files.py <gyp-flags>
-
-    For example: replace_gyp_files.py -Duse_system_harfbuzz=1
-
-    The script ignores flags other than -D for convenience. This makes it
-    possible to have a variable e.g. ${myconf} with all the options, and
-    execute:
-
-    build/linux/unbundle/replace_gyp_files.py ${myconf}
-    build/gyp_chromium ${myconf}
-
-2b. replace_gn_files.py --system-libraries lib...
-
-    For example: replace_gn_files.py --system-libraries libxml
+   For example: replace_gn_files.py --system-libraries libxml