Remove a waccess call when completing executables

[faho]

When we run these completions, we have already run waccess with X_OK. We already know the file is executable.

There is no reason to check again.

Restores some of the speedup from the fast_waccess hack that was removed to fix #9699.

In my tests it's ~1.14x - if fish with c67d77f reverted takes 1x, this takes 1.2 times as long and fish right before this takes 1.37x as long. (vs 3.6.1 it's 1.22x reverted / 1.5x this / 1.7x master)

The absolute time difference will scale with the size of $PATH.

Note: This will conflict with any port of wildcard, like #9916. I would still like to merge this because it should be in 3.6.2 and offset some of the loss of #9699.

I'm aware that this adds another argument to an already argument-heavy function, but it's only called in one spot and I would leave that cleanup to after a port (a lazy "file information" object would be quite cool to have here, but might be awkward considering TOCTTOU issues, but at the very least the "stat_res" and "stat buf" arguments would just be some combined thing like an Option or Result).

TODOs:

• [N/A] Changes to fish usage are reflected in user documentation/manpages.
• [N/A] Tests have been added for regressions fixed
• [N/A] User-visible changes noted in CHANGELOG.rst

[henrikhorluck]

I'm fine with adding the parameter, that function will have way fewer parameters in Rust

[zanchey]

Note for me - if we're taking this for 3.6.2 we will need to take #9699 as well