stm32g0: flash: drop redundant docs

Functions that are already documented in the top level common api.h file
won't add any more documentation from later .c files.  Keep docs for
part specifics, in the .h files where they're accessible to IDEs and
also the documentation generation, and drop all (including the redundant
ones) from the .c file.

include/libopencm3/stm32/g0/flash.h

@@ -178,13 +178,19 @@
 
 BEGIN_DECLS
 
+/** Enable instruction cache */
 void flash_icache_enable(void);
+/** Disable instruction cache */
 void flash_icache_disable(void);
+/** Reset instruction cache */
 void flash_icache_reset(void);
 
+/** Unlock program memory */
 void flash_unlock_progmem(void);
+/** lock program memory */
 void flash_lock_progmem(void);
 
+/** Lock Option Byte Access */
 void flash_lock_option_bytes(void);
 
 END_DECLS