makesyscalls.lua replaced all in-tree sysent targets back in
f22a5921112a ("Convert in-tree sysent targets to use new [...]"). A
deprecation notice was formally added in
bb97350f2838 ("makesyscalls.sh: spit out a deprecation notice [...]").
Remove it now, as no complaints have surfaced in the interim. The
notice that was added specifically called it out as being removed before
FreeBSD 13, so this commit may or may not end up considered for MFC to
stable/13.