INSTALL: don't mention the "curl" executable at all

In 1d53f90ed97 (The "curl" executable is no longer required, 2008-06-15) the wording for requiring curl(1) was changed to the current "you might also want...". Mentioning the "curl" executable at all is just confusing, someone building git might want to use it to debug things, but they might also just use wget(1) or some other http client. The "curl" executable has the advantage that you might be able to e.g. reproduce a bug in git's usage of libcurl with it, but anyone going to those extents is unlikely to be aided by this note in INSTALL. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>