Makefile.m32: stop trying to build libcares.a [ci skip]

Makefile.m32: stop trying to build libcares.a [ci skip]

Before this patch, `lib/Makefile.m32` had a rule to build `libcares.a` in
`-cares`-enabled builds, via c-ares's own `Makefile.m32`. Committed in
2007 [1]. The commit message doesn't specifically address this particular
change. This logic comes from the times when c-ares was part of the curl
source tree, hence the special treatment.

This feature creates problems when building c-ares first, using CMake
and pointing `LIBCARES_PATH` to its install prefix, where `Makefile.m32`
is missing in such case. A sub-build for c-ares is undesired also when
c-ares had already been build via its own `Makefile.m32`.

To avoid the sub-build, this patch deletes its Makefile rule. After this
patch `libcares.a` needs to be manually built before using it in
`Makefile.m32`. Aligning it with the rest of dependencies.

[1] 46c92c0b806da041d7a5c6fb64dbcdc474d99b31

Reviewed-by: Daniel Stenberg
Closes #9169