Remove GYP_GENERATORS=ninja code from android envsetup.

build/gyp_chromium already makes GYP_GENERATORS default to ninja on linux
these days.

BUG=330631

Review URL: https://codereview.chromium.org/121023002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242576 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242576