Simplify dxsyn_lexize().

Simplify dxsyn_lexize().

There's no need to create and free a temporary copy of the input,
since str_tolower() is already able to cope with not-certainly-
nul-terminated input.  (Before v18, copying was needed because
this code used lowerstr(), but now we can do without.)

Author: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/19525-b0be8e4eb7dbaf07@postgresql.org