kbuild: remove unused baseprereq

Commit eea199b4 ("kbuild: remove unnecessary LEX_PREFIX and
YACC_PREFIX") removed the last users of this macro.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/Kbuild.include

@@ -23,10 +23,6 @@ depfile = $(subst $(comma),_,$(dot-target).d)
 # filename of target with directory and extension stripped
 basetarget = $(basename $(notdir $@))
 
-###
-# filename of first prerequisite with directory and extension stripped
-baseprereq = $(basename $(notdir $<))
-
 ###
 # Escape single quote for use in echo statements
 escsq = $(subst $(squote),'\$(squote)',$1)