Revert "config.mk: Add MK_VIMAGE knob"

This commit broke "make makeman" checks in github CI due to a lack of
option description files.  The split between VIMAGE and VIMAGE_SUPPORT
is not clearly justified and the code is broken because there is no
opt_vimage.h (it's in opt_global.h).

This reverts commit 22ca6db50f.

share/mk/src.opts.mk

@@ -183,7 +183,6 @@ __DEFAULT_YES_OPTIONS = \
     USB \
     UTMPX \
     VI \
-    VIMAGE \
     VT \
     WIRELESS \
     WPA_SUPPLICANT_EAPOL \
@@ -248,7 +247,6 @@ __LIBC_MALLOC_DEFAULT=	jemalloc
     NETGRAPH \
     PAM \
     TESTS \
-    VIMAGE \
     WIRELESS
 __DEFAULT_DEPENDENT_OPTIONS+= ${var}_SUPPORT/${var}
 .endfor