index-format.txt: remove outdated list of supported extensions

index-format.txt: remove outdated list of supported extensions

The first section of 'Documentation/technical/index-format.txt'
mentions that "Git currently supports cache tree and resolve undo
extensions", but then goes on, and in the "Extensions" section
describes not only these two, but six other extensions [1].

Remove this sentence, as it's misleading about the status of all those
other extensions.

Alternatively we could keep that sentence and update the list of
extensions, but that might well lead to a recurring issue, because
apparently this list is never updated when a new index extension is
added.

[1] Split index, untracked cache, FS monitor cache, end of index
    entry, index entry offset table and sparse directory entries.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>