csu: Remove unused constant

This is no longer used after commit
99282790b7d01ec3c4072621d46a0d7302517ad4 moved ELF notes from C to
assembly.

Reviewed by: kib
Fixes: 99282790b7d0 ("Remove the sed hack for ABI tag notes.")
Sponsored by: AFRL, DARPA
Differential Revision: https://reviews.freebsd.org/D54506
(cherry picked from commit 113dc9ed98874d15f4f43287dcb6d0f59633b19b)