Eliminate the vestigial comment "magical thingies" from intrpvar.h

Eliminate the vestigial comment "magical thingies" from intrpvar.h

The original comment "magical thingies" was added to perl.c by commit
8ebc5c0145d2e355 in Jan 1997. It is above code which frees 4 interpreter-
global SVs in perl_destruct.

The comment "magical thingies" was in intrpvar.h since the file was created
by commit 49f531dad558d800 on 29 Nov 1997. At that time, it was followed by
a block of 13 relevant interpreter global variables. However, by commit
d4cce5f1785350c2 (30 Nov 1997) all bar two were now in other places, mostly
in thrdvar.h. With the abolition of PL_formfeed, the comment now annotates
just one "magical" thingy, PL_basetime, which isn't even one of the SVs
freed at the analogous location in perl.c. Hence the comment adds no value.