Delete swis.s

[thomasdziedzic]

Problem: swis.s is unused
Solution: delete it

I don't know if there is an actual use case, I don't know enough about the history of this file to be come to a certain conclusion and I couldn't find any documentation on why this file is needed.

Vim is currently just shipping the file as source code as is, but I don't see it being used anywhere in the code.

All I could dig up is that some of the comments look like they reference some swi calls from risc os like Wimp_ReportError

I also couldn't find risc os as a supported platform, so this is why I decided to open this pull request.

[thomasdziedzic]

Looks like there might have been an unrelated error for appveyer under the normal environment:

From test_channel.vim:
Found errors in Test_call():
function RunTheTest[24]..Test_call[2]..<SNR>6_run_server[1]..RunServer line 17: Can't start test_channel.py: Expected False but got 1
TEST FAILURE 
NMAKE : fatal error U1077: 'if' : return code '0x1'
Stop.
Command exited with code 2

This error doesn't look related to my change from my point of view.

[chrisbra]

Yeah, I think the error from appveyor can be ignored. Looking at vim-ancient one can see, that the last time this file was modified was for Patch 6.0.227 (and that was only changing how to build it).