libc/i386/SYS.h: Remove unused LCALL macro

It appears that the only user of this macro was removed
with support for building a.out binaries in 2002 by commit
66422f5b7a1a6055f0b2358268eb902aab6e2e3e.

Reviewed by: imp, kib
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D42997