lib: sbi: Remove the SBI_ETRAP error code

The SBI_ETRAP error code was introduced only for doing trap
redirection in generic sbi_ecall_handler(). Now the trap
redirection is moved into sbi_ecall_legacy.c and SBI_ETRAP
error code is only used in this source file so let us remove
it.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>