[ng] cleanup: remove few VPATH rewrite workarounds

[ng] cleanup: remove few VPATH rewrite workarounds

They are not needed by GNU make, which performs no implicit VPATH
rewrites in its recipes.

* lib/am/lisp.am (install-%DIR%LISP): No need to "correct" possible
VPATH rewrites: GNU makes doesn't do them.  Accordingly, ...
* lib/am/inst-vars.am (am__vpath_adj_setup, am__vpath_adj): ... these
two  variables are no more needed: remove them.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>