__attribute__: remove redundant attribute declaration for git_die_config()

__attribute__: remove redundant attribute declaration for git_die_config()

The convention is to declare the function attribute to an extern
function together with its declaration in the header file, without
repeating the attribute declaration with its definition in the .c
source file (a file-scope static function declares its attribute
together with its definition in the .c file it is defined, as there
is no other place to do so).

The definition of git_die_config() in config.c did not follow the
convention and had its attribute declared with both its declaration
in the header and its definition in the .c source file.

Remove the one in the config.c to match everybody else.

Signed-off-by: Junio C Hamano <gitster@pobox.com>