soc: snps: nsim: arc_v: rmx: remove redudant compiler filtering

Remove CONFIG_SOC_SERIES_NSIM_ARC_V_RMX conditional check from
zephyr_compile_options_ifdef and apply all extensions
unconditionally when using arcmwdt compiler.

Since this CMakeLists.txt is already SoC-specific, the
additional config check is redundant.

Signed-off-by: Afonso Oliveira <afonsoo@synopsys.com>