[alg.foreach] Delete mistaken Complexity: element on for_each_n

This was presumably added via accidental copy-paste in the application
of P3179R9.

Author: tkoeppe

source/algorithms.tex

@@ -4920,10 +4920,6 @@
 \returns
 \tcode{first + n}.
 
-\pnum
-\complexity
-Applies \tcode{f} and \tcode{proj} exactly \tcode{last - first} times.
-
 \pnum
 \remarks
 \begin{itemize}