Remove ceres-solver from the manifest

It is not referenced by anything any more.
Test: m -j checkbuild
Change-Id: Id0e011527772c3ec1b491a74db5e19e0855977e8

default.xml

@@ -100,7 +100,6 @@
   <project path="external/c-ares" name="platform/external/c-ares" groups="pdk" />
   <project path="external/caliper" name="platform/external/caliper" groups="pdk" />
   <project path="external/cblas" name="platform/external/cblas" groups="pdk" />
-  <project path="external/ceres-solver" name="platform/external/ceres-solver" groups="pdk" />
   <project path="external/chromium-libpac" name="platform/external/chromium-libpac" groups="pdk" />
   <project path="external/chromium-trace" name="platform/external/chromium-trace" groups="pdk" />
   <project path="external/chromium-webview" name="platform/external/chromium-webview" groups="pdk" />