oidset: don't return value from oidset_init

oidset: don't return value from oidset_init

c3a9ad3117 ("oidset: add iterator methods to oidset", 2017-11-21)
introduced a 'oidset_init()' function in oidset.h, which has void as
return type, but returns an expression.

This makes the solaris compiler fail with:

    "oidset.h", line 30: void function cannot return value

As the return type is void, and even the return type of the expression
we're trying to return (oidmap_init) is void just remove the return
statement to fix the compiler error.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>