Remove obsolete strp library

This was not referenced since it was added. Remove it.
Test: make -j32
Bug: 34811035
Change-Id: I30e96a2c6c87e589b3deed42d189d34f379ce6cc

default.xml

@@ -299,7 +299,6 @@
   <project path="external/speex" name="platform/external/speex" groups="pdk" />
   <project path="external/sqlite" name="platform/external/sqlite" groups="pdk" />
   <project path="external/squashfs-tools" name="platform/external/squashfs-tools" groups="pdk" />
-  <project path="external/srtp" name="platform/external/srtp" groups="pdk" />
   <project path="external/strace" name="platform/external/strace" groups="pdk" />
   <project path="external/svox" name="platform/external/svox" groups="pdk" />
   <project path="external/tagsoup" name="platform/external/tagsoup" groups="pdk" />