[RISCV] Remove -riscv-v-fixed-length-vector-lmul-max from tests. NFC (#78299)

Some fixed vector tests in test/CodeGen/RISCV/rvv have multiple run
lines that
check various configurations of -riscv-v-fixed-length-vector-lmul-max.
From
what I understand this flag was introduced in the early days of fixed
length
vector support, but now that fixed vector codegen has matured I'm not
sure if
it's as relevant today.

This patch proposes to remove the various lmul-max run lines from the
tests to
make them more readable, and any changes to fixed vector codegen easier
to
review.

We have removed them before for the same reason, so this would take care
of the
remaining test cases: https://reviews.llvm.org/D157973#4593268

(I don't have any strong motivation to remove the actual flag itself, my
own
personal motivation is just to clean up the tests)