exec: Remove an unneeded capability mode check

[markj]

Summary

The subsequent namei() call is relative to AT_FDCWD, and such lookups
are always disallowed in capability mode.

No functional change intended.

[emaste]

For historical context, introduced in rG:12bc222e5720dd2c4672a119f56024a86775414d. This change is indeed fine because "we are sufficiently confident that the checks in namei() are correct." If they were not, we have bigger problems.