[RISCV] Remove is*Branch instruction flags from C_JR

As discussed in 1d1b3c49, instruction flags set in the *.td files
are under-approximations. For C_JR, isBranch and isConditionalBranch are
set even though it is used for for returns which are not considered
branches.

This patch proposes to remove those flags from C_JR. More detailed
analysis can be implemented in RISCVMCInstrAnalysis.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D147784