csu: Remove unused constant

This is no longer used after commit
99282790b7 moved ELF notes from C to
assembly.

Reviewed by:	kib
Fixes:		99282790b7 ("Remove the sed hack for ABI tag notes.")
Sponsored by:	AFRL, DARPA
Differential Revision:	https://reviews.freebsd.org/D54506

lib/csu/common/notes.h

@@ -27,6 +27,4 @@
 
 #define NOTE_FREEBSD_VENDOR	"FreeBSD"
 
-#define NOTE_SECTION		".note.tag"
-
 #endif