math/R: remove obsolete fpsetmask

src/unix/sys-unix.c uses fpsetmask to workaround a bug that is not present since FreeBSD 4.0.

This issue also affected Firefox and was resolved 9 years ago https://bugzilla.mozilla.org/show_bug.cgi?id=789436#c27

fpsetmask executions cause R build to crash on powerpc64(le), since new SIGFPE kernel code doesn't take into account old workarounds.

PR: 254621
Approved by: jrm (maintainer)