QA-945: Remove now unused testing image for mender-gateway

[lluiscampos]

A follow-up from where we removed the use of this image in integration tests suites.

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[lluiscampos]

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Build Configuration Matrix

Key	Value
BUILD_BEAGLEBONEBLACK	true
BUILD_CLIENT	true
BUILD_QEMUX86_64_BIOS_GRUB	true
BUILD_QEMUX86_64_BIOS_GRUB_GPT	true
BUILD_QEMUX86_64_UEFI_GRUB	true
BUILD_VEXPRESS_QEMU	true
BUILD_VEXPRESS_QEMU_FLASH	true
BUILD_VEXPRESS_QEMU_UBOOT_UEFI_GRUB	true
META_MENDER_REV	pull/2307/head
RUN_INTEGRATION_TESTS	true
TEST_QEMUX86_64_BIOS_GRUB	true
TEST_QEMUX86_64_BIOS_GRUB_GPT	true
TEST_QEMUX86_64_UEFI_GRUB	true
TEST_VEXPRESS_QEMU	true
TEST_VEXPRESS_QEMU_FLASH	true
TEST_VEXPRESS_QEMU_UBOOT_UEFI_GRUB	true