fstests: btrfs/080: fix the stray '\'

fstests: btrfs/080: fix the stray '\'

[BUG]
The latest grep will report stray '\', causing golden output mismatch
for btrfs/080:

btrfs/080       - output mismatch (see ~/xfstests-dev/results//btrfs/080.out.bad)
    --- tests/btrfs/080.out	2022-11-24 19:53:53.137469203 +0800
    +++ ~/xfstests-dev/results//btrfs/080.out.bad	2022-12-09 11:41:46.194597311 +0800
    @@ -1,2 +1,3 @@
     QA output created by 080
    +grep: warning: stray \ before -
     Silence is golden
    ...
    (Run 'diff -u ~/xfstests-dev/tests/btrfs/080.out ~/xfstests-dev/results//btrfs/080.out.bad'  to see the entire diff)

[CAUSE]
Even for regrex of grep, '-' doesn't need special escape, thus
"\bno\-holes\b" indeed has an unnecessary '\' before '-'.

[FIX]
Just remove the stray '\'.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>