arm64: Remove unused VGA console support

Support for VGA_CONSOLE is not allowable due to commit ee23794b
("video: vgacon: Don't build on arm64"), thus remove the associated
unused code.

Whilst PCI on arm64 would support VGA a valid screen_info structure
is missing.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Andrew Murray <andrew.murray@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>