Remove external/ltrace from the manifest.

I've fixed ltrace, but I've also determined that it now builds out of
the box anyway. Plus no-one complained for the whole year that the makefile
was disabled.
Bug: http://b/19715557
Change-Id: I8b8ef7faaf5f2c4328f7eea056cb0b1e9414758e

default.xml

@@ -219,7 +219,6 @@
   <project path="external/littlemock" name="platform/external/littlemock" groups="pdk" />
   <project path="external/lld" name="platform/external/lld" groups="pdk" />
   <project path="external/llvm" name="platform/external/llvm" groups="pdk" />
-  <project path="external/ltrace" name="platform/external/ltrace" groups="pdk" />
   <project path="external/lz4" name="platform/external/lz4" groups="pdk" />
   <project path="external/lzma" name="platform/external/lzma" groups="pdk" />
   <project path="external/markdown" name="platform/external/markdown" groups="pdk" />