[gdb/contrib] Remove superfluous .alt file after dwz invocation in cc-with-tweaks.sh

[gdb/contrib] Remove superfluous .alt file after dwz invocation in cc-with-tweaks.sh

The -m option of cc-with-tweaks.sh sets want_multi to true, invoking dwz like
this:
...
elif [ "$want_multi" = true ]; then
    cp $output_file ${output_file}.alt
    $DWZ -m ${output_file}.dwz "$output_file" ${output_file}.alt \
        > /dev/null 2>&1
fi
...

The problem that is being solved here, is that we want to test dwz in
multifile mode, which requires more than one input file, while we only have
(at the scope of cc-with-tweaks.sh) one executable.  We handle this by copying
the executable and offering this as a second input (and using a copy has the
additional benefit that it maximally enables dwz transformation).

However, after the dwz invocation, the copy is no longer used, and the
presence of the file actually causes a test regression:
...
FAIL: gdb.base/jit-so.exp: test jit-reader-load filename completion
...

Fix this by removing the superflous copy after dwz invocation.

Tested on x86_64-linux.

gdb/ChangeLog:

2019-04-23  Tom de Vries  <tdevries@suse.de>

	PR gdb/24438
	* contrib/cc-with-tweaks.sh: Remove superfluous .alt file after dwz
	invocation.