meson: Drop legacy 'verify-signature' option

Currently there are two options for the image signature verification:
- 'verify-signature' (legacy),
- 'verify-full-signature' (current).

Drop legacy option and let the current one take its place.

Change-Id: Ia205424395179fd190e3d055159beca93cff0545
Signed-off-by: Konstantin Aladyshev <aladyshev22@gmail.com>
diff --git a/image_verify.cpp b/image_verify.cpp
index 1c81abe..bfdab62 100644
--- a/image_verify.cpp
+++ b/image_verify.cpp
@@ -100,7 +100,7 @@
 bool Signature::verifyFullImage()
 {
     bool ret = true;
-#ifdef WANT_SIGNATURE_FULL_VERIFY
+#ifdef WANT_SIGNATURE_VERIFY
     // Only verify full image for BMC
     if (purpose != VersionPurpose::BMC)
     {