Remove __restrict qualifiers from syscalls.master.

The restruct qualifier is intended to aid code generation in the
compiler, but the only access to storage through these pointers is via
structs using copyin/copyout and the like which can not be written in C
or C++ and thus the compiler gains nothing from the qualifiers.

As such, the qualifiers add no value in current usage.

Reviewed by: kib
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D17574