bootstrap: No need to disable shared libraries for bootstrap tools

There's no need to disable shared libraries when building the bootstrap
tools. This was added on 2000 (commit ad879ce9552c) when the perl
bootstrap was added (libperl and miniperl) and saved a fair amount of
time (perl took a long time to build on 2000-era hardware).

For many years now, however, we rarely build any libraries when
bootstrapping. Even when we do, the optimization saves at most a few
seconds when upgrading since the libraries built have been small. Shared
libraries are more robust accross versions that static libraries due to
creaping dependencies (we aren't crossing versions of share libraries,
though, just using what's on the host). In addition, linux and macos
have been building like this for some time because static binaries on
those systems are difficult to impossible.

Sponsored by: Netflix
Reviewed by: arichardson, bapt
Differential Revision: https://reviews.freebsd.org/D32443