makesyscalls: rip out arbitrary command execution

This was previously needed only for CloudABI, which used it to generate
its capenabled from syscalls.master. CloudABI was removed in
cf0ee8738e31, so we don't need to support this anymore. Others looking
to do similar things should come up with a more integrated technique,
such as a .conf flag or pattern/glob support. brooks suggests that it
could be done in modern makesyscalls.lua by adding a config flag to
specify always-on/initial flags (CAPENABLED).

Reviewed by: brooks, imp
MFC after: never
Differential Revision: https://reviews.freebsd.org/D32095