[Android] envsetup: Stop unsetting CHROMIUM_GYP_FILE.

This is one step towards simplifying our envsetup.sh. Chromium stopped
touching CHROMIUM_GYP_FILE in r248748, and it is safe for us to do the
same.
Merge pull request #1874 from rakuco/no-unset-chromium_gyp_file

[Android] envsetup: Stop unsetting CHROMIUM_GYP_FILE.