tools/ci.sh: Remove explicit macOS pkg-config install.

Reasons to remove this:

- GitHub's macOS runners install this package by default nowadays.

- Brew renamed this package to 'pkgconf' so installing the old name on top
  of the new package name has started failing.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

tools/ci.sh

@@ -651,9 +651,6 @@ function ci_unix_settrace_stackless_run_tests {
 }
 
 function ci_unix_macos_build {
-    # Install pkg-config to configure libffi paths.
-    brew install pkg-config
-
     make ${MAKEOPTS} -C mpy-cross
     make ${MAKEOPTS} -C ports/unix submodules
     #make ${MAKEOPTS} -C ports/unix deplibs