drivers/firmware: Make efi/esrt.c driver explicitly non-modular

The Kconfig for this driver is currently hidden with:

config EFI_ESRT
        bool

...meaning that it currently is not being built as a module by anyone.
Lets remove the modular code that is essentially orphaned, so that
when reading the driver there is no doubt it is builtin-only.

Since module_init translates to device_initcall in the non-modular
case, the init ordering remains unchanged with this commit.

We leave some tags like MODULE_AUTHOR for documentation purposes.

We don't replace module.h with init.h since the file already has that.

Cc: Peter Jones <pjones@redhat.com>
Cc: linux-efi@vger.kernel.org
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>