Drop libtool vestiges

Long ago (prior to 91262e75) we used libtool, a tool intended to abstract over platform differences in linking behavior, for build of static libraries. However, in classic Apple fashion, macOS ships a different libtool implementation which is apparently incompatible with GNU libtool. For this reason, 91262e75 dropped our usage of libtool in favor of direct invocations of ar and ranlib. However, the (now-dead) code implementing the libtool path was left in place.

Here we drop this code, along with the one remaining trace of libtool support in gendll, allowing us to drop all of our libtool logic.

This is based on !7480 (merged).

Closes #18826 (closed).