doc: Remove empty "poll" section

In commit 231b8f9f, when porting from doxygen to gtk-doc, we created a
section for "poll" functions, but we renamed that section to "events"
during the porting process. We ended up with an empty "poll" section,
and a correctly migrated "events" section.

doc/libfprint-sections.txt

@@ -139,11 +139,6 @@ fp_img_get_minutiae
 fp_minutia_get_coords
 </SECTION>
 
-<SECTION>
-<INCLUDE>fprint.h</INCLUDE>
-<FILE>poll</FILE>
-</SECTION>
-
 <SECTION>
 <INCLUDE>fpi-log.h</INCLUDE>
 <FILE>fpi-log</FILE>