efi_loader: capsule: Remove the check for capsule_authentication_enabled environment variable

The current capsule authentication code checks if the environment
variable capsule_authentication_enabled is set, for authenticating the
capsule. This is in addition to the check for the config symbol
CONFIG_EFI_CAPSULE_AUTHENTICATE. Remove the check for the environment
variable. The capsule will now be authenticated if the config symbol
is set.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviwed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>