Remove unnecessary set of PDK environment variable

Latest Openlane is failing with this for some reason.

openlane/user_project_wrapper/config.tcl

diff --git a/openlane/user_project_wrapper/config.tcl b/openlane/user_project_wrapper/config.tcl
index 772cae7..891ce8b 100755
--- a/openlane/user_project_wrapper/config.tcl
+++ b/openlane/user_project_wrapper/config.tcl
@@ -16,7 +16,6 @@
 # Base Configurations. Don't Touch
 # section begin
 
-set ::env(PDK) $::env(PDK)
 set ::env(STD_CELL_LIBRARY) "sky130_fd_sc_hd"
 
 # YOU ARE NOT ALLOWED TO CHANGE ANY VARIABLES DEFINED IN THE FIXED WRAPPER CFGS