efi_loader: Cleanup get_var duplication

get_var() is defined statically in efi_bootmgr.c and doesn't properly
check a buffer allocation.  Remove it completely and use the exported
function from efi_var_common.c that does the same thing

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>