tests: riscv: Remove local z_riscv_pmp_read_config() prototype in PMP test

The function prototype for `z_riscv_pmp_read_config()` was previously
declared in `tests/arch/riscv/pmp/clear-pmp-unlocked-entries/src/main.c`.

This local declaration is no longer necessary as the prototype is now
defined within the centralized PMP header, `include/arch/riscv/pmp.h`,
which is already included via `kernel_internal.h`. The prototype
is guarded by the `CONFIG_ZTEST` Kconfig option.

Signed-off-by: Firas Sammoura <fsammoura@google.com>