Don't declare tui_init_content_element

Don't declare tui_init_content_element

I noticed that tui_init_content_element is declared but never defined.
This removes the declaration.  Tested by rebuilding.  (I should have
merged this with the previous patch but I had forgotten that I found
two of these.)

gdb/ChangeLog
2019-06-19  Tom Tromey  <tom@tromey.com>

	* tui/tui-data.h (tui_init_content_element): Don't declare.