[RISCV] Remove isCall instruction flag from JAL/JALR

As far as I can tell, the instruction flags set in the various
RISCVInstrInfo*.td files are always under-approximations. That is,
something like isTerminator will only be set for opcodes that are
*always* terminators.

The exception seems to be isCall; this is set for JAL/JALR while these
instruction are not always calls. This patch proposes to remove isCall
from JAL/JALR for consistency.

A follow-up patch will add more detailed analysis of some instruction
properties based on MCInstrAnalysis.

Reviewed By: asb, craig.topper

Differential Revision: https://reviews.llvm.org/D146437