bionic: libc: remove -Wimplicit-fallthrough

aosp/144287300 set it to a global cppflag.

The compiler upgrade can now check for this warning in C code.

This patch should be reverted once the BSD sources with instances of
-Wimplicit-fallthrough have been fixed.

Remove it from cflags, so that it's not re-enabled for C code until
fixed.

Bug: 139945549
Bug: 144287300
Test: mm
Change-Id: Ieca0d5b41634636477392e5209a41807f9b44bd4
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
diff --git a/libc/Android.bp b/libc/Android.bp
index c5ea4c5..9c2d713 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -42,7 +42,6 @@
     "-Wno-deprecated-declarations",
     "-Wno-gcc-compat",
     "-Wframe-larger-than=2048",
-    "-Wimplicit-fallthrough",
 
     // Try to catch typical 32-bit assumptions that break with 64-bit pointers.
     "-Werror=pointer-to-int-cast",