Remove google-diff-match-patch

This project was added in 2010 with no updates and the diff_match_patch
module doesn't seem to referenced by any current makefile.
Change-Id: Ia42b5ae0639545feb803660607ae56b635235121

default.xml

@@ -139,7 +139,6 @@
   <project path="external/genext2fs" name="platform/external/genext2fs" groups="pdk-cw-fs" />
   <project path="external/giflib" name="platform/external/giflib" groups="pdk-cw-fs,qcom_msm8x26" />
   <project path="external/glide" name="platform/external/glide" />
-  <project path="external/google-diff-match-patch" name="platform/external/google-diff-match-patch" groups="pdk-cw-fs" />
   <project path="external/google-fonts/coming-soon" name="platform/external/google-fonts/coming-soon" groups="pdk-cw-fs" />
   <project path="external/google-fonts/dancing-script" name="platform/external/google-fonts/dancing-script" groups="pdk-cw-fs" />
   <project path="external/google-fonts/carrois-gothic-sc" name="platform/external/google-fonts/carrois-gothic-sc" groups="pdk-cw-fs" />