Remove some legacy declarations from <math.h>.

These are still needed for backwards compatibility with code built by old
versions of the NDK, but we don't need to pollute the headers with them.

Also lose the hand-written code for these. The compiler-generated code
is either the same or better, and no new code is calling these functions
anyway.

Bug: N/A
Test: ran tests
Change-Id: Ib01ad9805034433e0105aec882608cc8e6526f78
diff --git a/libm/Android.bp b/libm/Android.bp
index bf86264..da9c9a8 100644
--- a/libm/Android.bp
+++ b/libm/Android.bp
@@ -210,11 +210,9 @@
         "sincos.c",
         "fake_long_double.c",
 
-        // Modified versions of BSD code.
-        "signbit.c",
-
         // Home-grown stuff.
         "fabs.cpp",
+        "signbit.cpp",
     ],
 
     multilib: {