rtc: make class.c explicitly non-modular

The Makefile/Kconfig currently controlling compilation of this code is:

obj-$(CONFIG_RTC_CLASS)         += rtc-core.o
rtc-core-y                      := class.o interface.o

drivers/rtc/Kconfig:menuconfig RTC_CLASS
drivers/rtc/Kconfig:    bool "Real Time Clock"

...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 code there is no doubt it is builtin-only.

We don't replace module.h with init.h since the file does need
to know what a struct module is.

We also delete the MODULE_LICENSE tag etc. since all that information
is already contained at the top of the file in the comments.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>