remove incorrect comment regarding powl exceptional cases

the comment does not match the required or actual behavior when x<0
and y is not an integer. while it could be corrected, the role of
comments here is to tell about characteristics unique to the
implementation, not to restate the requirements of the standard, so
just removing it seems best.

src/math/powl.c

@@ -57,14 +57,6 @@
  *    IEEE     0,8700       60000      6.5e-18      1.0e-18
  * 0.99 < x < 1.01, 0 < y < 8700, uniformly distributed.
  *
- *
- * ERROR MESSAGES:
- *
- *   message         condition      value returned
- * pow overflow     x**y > MAXNUM      INFINITY
- * pow underflow   x**y < 1/MAXNUM       0.0
- * pow domain      x<0 and y noninteger  0.0
- *
  */
 
 #include "libm.h"