boards/hifive1: remove erroneous features parsing

Remove duplicate handling of `periph_rtc` dependency to `periph_rtt` now
declared in the cpu.

Also the makefiles should not parse FEATURES_REQUIRED as it does not
guarantee anything as features could also be requested by FEATURES_OPTIONAL.
In practice it should parse it using `USEMODULE`.

boards/hifive1/Makefile.features

@@ -8,9 +8,5 @@ FEATURES_PROVIDED += periph_rtt
 FEATURES_PROVIDED += periph_timer
 FEATURES_PROVIDED += periph_uart
 
-ifneq (,$(filter periph_rtc,$(FEATURES_REQUIRED)))
-  FEATURES_REQUIRED += periph_rtt
-endif
-
 # The board MPU family (used for grouping by the CI system)
 FEATURES_MCU_GROUP = risc_v