Commit f38646593c (Allow `export` to set colon-separated `PATH`, `CDPATH`
and `MANPATH`., 2017-02-10)
did something very weird for «export PATH=foo».
It essentially does
set -gx PATH (string replace -- "$PATH" (string join ":" -- $PATH) foo)
which makes no sense. It should set PATH to "foo", no need to involve the
existing value of $PATH.
Additionally, the string split / string join dance is unnecessary. Nowadays,
builtin set already handles path variables as is needed here, so get rid of
this special case.
Fixes#11434
