build: remove unused legacy gzip helpers

Image/Gzip lost its last user with commit df9ffedb75 ("archs38: drop
target") and Image/gzip-ext4-padded-squashfs with commit 8dfa38b82c
("malta: convert to Device macro image building"). The latter also
calls Image/Build/gzip, which isn't defined anywhere.

Remove both, along with Image/pad-root-squashfs, which was only used by
Image/gzip-ext4-padded-squashfs.

Link: https://github.com/openwrt/openwrt/pull/25658
Signed-off-by: Paul Spooren <mail@aparcar.org>

include/image.mk

@@ -185,14 +185,6 @@ define Image/BuildKernel/MkuImage
 		-n '$(call toupper,$(ARCH)) $(VERSION_DIST) Linux-$(LINUX_VERSION)' -d $(4) $(5)
 endef
 
-ifdef CONFIG_TARGET_IMAGES_GZIP
-  define Image/Gzip
-	rm -f $(1).gz
-	gzip -9n $(1)
-  endef
-endif
-
-
 # Disable noisy checks by default as in upstream
 DTC_WARN_FLAGS := \
   -Wno-interrupt_provider \
@@ -221,10 +213,6 @@ ifeq ($(DUMP),)
 ROOTFS_PARTSIZE=$(shell echo $$(($(CONFIG_TARGET_ROOTFS_PARTSIZE)*1024*1024)))
 endif
 
-define Image/pad-root-squashfs
-	$(call Image/pad-to,$(KDIR)/root.squashfs,$(if $(1),$(1),$(ROOTFS_PARTSIZE)))
-endef
-
 # $(1) source dts file
 # $(2) target dtb file
 # $(3) extra CPP flags
@@ -357,23 +345,6 @@ ifneq ($(CONFIG_JSON_CYCLONEDX_SBOM),)
 endif
 endef
 
-define Image/gzip-ext4-padded-squashfs
-
-  define Image/Build/squashfs
-    $(call Image/pad-root-squashfs)
-  endef
-
-  ifneq ($(CONFIG_TARGET_IMAGES_GZIP),)
-    define Image/Build/gzip/ext4
-      $(call Image/Build/gzip,ext4)
-    endef
-    define Image/Build/gzip/squashfs
-      $(call Image/Build/gzip,squashfs)
-    endef
-  endif
-
-endef
-
 ifeq ($(filter-out targz,$(ROOTFS_FILESYSTEM)),)
 ifdef CONFIG_TARGET_ROOTFS_TARGZ
   define Image/Build/targz