dtrace: Remove the noinline qualifier from dtrace_getarg()

It serves no purpose after commit 82283cad12a4. No functional change
intended.

Fixes: 82283cad12a4 ("dtrace: Avoid including dtrace_isa.c directly into dtrace.c")
MFC after: 2 weeks

(cherry picked from commit 494a2b14722e8955af54e306305f462e5765c1e4)