On RISC-V we can't store integers in floating-point registers as this is
forbidden by the ISA.  We've always disallowed this, but we were
setting the preferred mode to FP_REGS for some integer modes.  This
caused the LRA to blow up with some hard to read error messages.

This patch removes the prefered mode hook, as the right thing to do here
is nothing.

Thanks to Kito for finding the bug, and mpf for the fix.  See also
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79912>.

PR target/79912

From-SVN: r246283