lib/reg_script.c: Remove unused assignment

Fix the warning below.

```
    CC         bootblock/lib/reg_script.o
src/lib/reg_script.c:375:11: warning: Value stored to 'value' during its \
initialization is never read
        uint64_t value = msr.hi;
                 ^~~~~   ~~~~~~
```

Found-by: Clang static analyzer scan-build
          (clang version 4.0.1-6 (tags/RELEASE_401/final))
Fixes: fd461e39 (regscript: Add support for MSR type)
Change-Id: I218e45d12f2f00e2ad4cfe5410029f407b57568d
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-on: https://review.coreboot.org/22197
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>

src/lib/reg_script.c

@@ -373,7 +373,6 @@ static uint64_t reg_script_read_msr(struct reg_script_context *ctx)
 	const struct reg_script *step = reg_script_get_step(ctx);
 	msr_t msr = rdmsr(step->reg);
 	uint64_t value = msr.hi;
-	value = msr.hi;
 	value <<= 32;
 	value |= msr.lo;
 	return value;