riscv: sbi: Remove sbi_spec_version

U-Boot defaults to use SBI v0.2. Howerver there is a global variable
sbi_spec_version that stills refers to v0.1. Since it is not used
anywhere, let's remove it.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Rick Chen <rick@andestech.com>