flann: Remove gtest dependency

gtest was removed because it is not meant to be installed on its own,
but rather included as a part of software build systems. flann was the
only formula which expected a preinstalled copy of gtest; this disables
its tests which depend on gtest.

Fixes Homebrew/homebrew#14126.

Formula/flann.rb

@@ -14,7 +14,6 @@ class Flann < Formula
   end
 
   depends_on 'cmake' => :build
-  depends_on 'gtest' => :build
   depends_on 'hdf5'
 
   depends_on 'octave' if ARGV.include? '--enable-matlab'