doc: remove STRIP_FROM_INC_PATH

To be able to include *_params.h in docuementation, STRIP_FROM_INC_PATH has to be empty. Otherwise, a number of warnings are thrown if *_params.h files with same names exist in different directories.

doc/doxygen/riot.doxyfile

@@ -161,7 +161,7 @@ STRIP_FROM_PATH        =
 # specify the list of include paths that are normally passed to the compiler
 # using the -I flag.
 
-STRIP_FROM_INC_PATH    = $(STRIP_FROM_INC_PATH_LIST) # Exported from Makefile.
+STRIP_FROM_INC_PATH    = # $(STRIP_FROM_INC_PATH_LIST) # Exported from Makefile.
 
 # If the SHORT_NAMES tag is set to YES, doxygen will generate much shorter (but
 # less readable) file names. This can be useful is your file systems doesn't