Remove extra checkbuild argument from ninja tools command lines

The dangling rules check was running `ninja checkbuild -t targets rule`.
This apparently worked fine with glibc, but is failing when ninja is
compiled with musl.  Musl's getopt_long is leaving "checkbuild" in argv
when calling ninja's ToolTargets, resulting in:
ninja: error: unknown target tool mode 'checkbuild'

Remove the extraneous checkbuild argument.

Bug: 190084016
Test: m checkbuild
Change-Id: I61b6475a5f7cb9d3d47232057ecfdc798864dd81