test-lib: drop comment about test_description

test-lib: drop comment about test_description

When a comment describing how each test file should start was added in
commit [1], it was the second comment of t/test-lib.sh.  The comment
describes how variable "test_description" is supposed to be assigned at
the top of each test file and how "test-lib.sh" should be used by
sourcing it.  However, even in [1], the comment was ten lines away from
the usage of the variable by test-lib.sh.  Since then, the comment has
drifted away both from the top of the file and from the usage of the
variable.  The comment just sits in the middle of the initialization of
the test library, surrounded by unrelated code, almost one hundred lines
away from the usage of "test_description".

Nobody has noticed this drift during evolution of test-lib.sh, which
suggests that this comment has outlived its usefulness.  The assignment
of "test_description", sourcing of "test-lib.sh" by tests, and the
process of writing tests in general are described in detail in
"t/README".  So drop the obsolete comment.

An alternative solution could be to move the comment either to the top
of the file, or down to the usage of variable "test_description".

[1] e1970ce43a ("[PATCH 1/2] Test framework take two.", 2005-05-13)

Signed-off-by: Andrei Rybak <rybak.a.v@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>