remove cargo culted redundant PL_perldb test

Inseperable changes commit 8490252049 and specifically ML post
"Perldb rehaul" by	Ilya Zakharevich added a completely redundant
"if(PL_perldb)" test. This was probably cargo culted from the code prior
to that patch where PL_perldb was a (I think) a bool. Neither GCC 4.6.3
nor VC 2003  optimized away the test. Before this patch, .text section in
perl521.dll compiled with VC 2003 was 0xc62d3 bytes of machine code long,
after 0xc6203.
remove obsolete PL_apiversion

Commit 0e42d607f5 made PL_apiversion unused. Remove it to save memory in
interp struct.