Clean up comments and whitespace characters in source files

Remove all remaining "last changed" version info from source comments.
(The version control system maintains this information automatically.)

Delete the trailing whitespace characters.

contrib/arm-neon/android-ndk.c

@@ -2,7 +2,6 @@
  *
  * Copyright (c) 2014 Glenn Randers-Pehrson
  * Written by John Bowler, 2014.
- * Last changed in libpng 1.6.10 [March 6, 2014]
  *
  * This code is released under the libpng license.
  * For conditions of distribution and use, see the disclaimer
@@ -24,6 +23,7 @@
  * has been compiled only, not linked: no version of the library has been found,
  * only the header files exist in the NDK.
  */
+
 #include <cpu-features.h>
 
 static int

contrib/arm-neon/linux-auxv.c

@@ -2,7 +2,6 @@
  *
  * Copyright (c) 2014 Glenn Randers-Pehrson
  * Written by Mans Rullgard, 2011.
- * Last changed in libpng 1.6.10 [March 6, 2014]
  *
  * This code is released under the libpng license.
  * For conditions of distribution and use, see the disclaimer
@@ -22,6 +21,7 @@
  * This generic __linux__ implementation requires reading /proc/self/auxv and
  * looking at each element for one that records NEON capabilities.
  */
+
 #include <unistd.h> /* for POSIX 1003.1 */
 #include <errno.h>  /* for EINTR */
 

contrib/arm-neon/linux.c

@@ -1,6 +1,5 @@
 /* contrib/arm-neon/linux.c
  *
- * Last changed in libpng 1.6.31 [July 27, 2017]
  * Copyright (c) 2014, 2017 Glenn Randers-Pehrson
  * Written by John Bowler, 2014, 2017.
  *
@@ -19,6 +18,7 @@
  * This code is strict ANSI-C and is probably moderately portable; it does
  * however use <stdio.h> and it assumes that /proc/cpuinfo is never localized.
  */
+
 #include <stdio.h>
 
 static int