lang/go14: Don't install doc and test files

Currently, lang/go14 installs about 1,400-ish doc and test files.
The only role of go14 is to build lang/go, and those files have
no role in that.

PR: 239857
Approved by: jlaffaye (maintainer timeout, >1 month), tz (mentor)
Differential Revision: https://reviews.freebsd.org/D21879