arch/riscv: remove the `Kconfig.core` file

This commit removes the `Kconfig.core` file. It's been largely unused, and
the only symbol it provides (`RISCV_CORE_E31`) overlaps with the SoC-layer
provided `SOC_SERIES_SIFIVE_FREEDOM_FE300`.

As of date, the only SoC that uses the E31 core in Zephyr is the FE310 SoC.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>