Remove unuseful variable 'PD_PATH'
Remove unuseful variable 'PD_PATH'

The variable was introduced in commit 41e9743 for compatibility
with centralized build system as inherited from Pd-extended by
Pd-L2ork and Purr Data. But it doesn't work in that context because
$(PD_PATH)/bin is absent. Therefore better remove 'PD_PATH' and
focus on making variables 'PDDIR' & Co. work as expected.

myQwil/pd-externals