tests: riscv Remove redundant z_riscv_pmp_read_config declaration

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
conditionally defined within the centralized PMP header,
`include/arch/riscv/pmp.h`, guarded by the `CONFIG_ZTEST` Kconfig option:

Signed-off-by: Firas Sammoura <[email protected]>

Author: fsammoura1980

tests/arch/riscv/pmp/clear-pmp-unlocked-entries/src/main.c

@@ -7,8 +7,6 @@
 #include <zephyr/tc_util.h>
 #include <zephyr/ztest.h>
 
-void z_riscv_pmp_read_config(unsigned long *pmp_cfg, size_t pmp_cfg_size);
-
 /* Checks if the Machine Privilege Register Virtualization (MPRV) bit in mstatus is 1 (enabled). */
 static bool riscv_mprv_is_enabled(void)
 {