gc docs: remove incorrect reference to gc.auto=0

The chance of a repository being corrupted due to a "gc" has nothing
to do with whether or not that "gc" was invoked via "gc --auto", but
whether there's other concurrent operations happening.

This is already noted earlier in the paragraph, so there's no reason
to suggest this here. The user can infer from the rest of the
documentation that "gc" will run automatically unless gc.auto=0 is
set, and we shouldn't confuse the issue by implying that "gc --auto"
is somehow more prone to produce corruption than a normal "gc".

Well, it is in the sense that a blocking "gc" would stop you from
doing anything else in *that* particular terminal window, but users
are likely to have another window, or to be worried about how
concurrent "gc" on a server might cause corruption.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-gc.txt

@@ -141,8 +141,7 @@ mitigate this problem:
 
 However, these features fall short of a complete solution, so users who
 run commands concurrently have to live with some risk of corruption (which
-seems to be low in practice) unless they turn off automatic garbage
+seems to be low in practice).
-collection with 'git config gc.auto 0'.
 
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