Fix obsolete comments in instr_time.h.

Commit 623cc673 removed gettimeofday(), and commits 24c3ce8f and
495ed0ef removed support for very old Windows releases with low accuracy
timers, but references to those things were left behind in comments.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/295419.1659918447%40sss.pgh.pa.us

src/include/portability/instr_time.h

@@ -4,10 +4,9 @@
  *	  portable high-precision interval timing
  *
  * This file provides an abstraction layer to hide portability issues in
- * interval timing.  On Unix we use clock_gettime() if available, else
- * gettimeofday().  On Windows, gettimeofday() gives a low-precision result
- * so we must use QueryPerformanceCounter() instead.  These macros also give
- * some breathing room to use other high-precision-timing APIs.
+ * interval timing.  On Unix we use clock_gettime(), and on Windows we use
+ * QueryPerformanceCounter().  These macros also give some breathing room to
+ * use other high-precision-timing APIs.
  *
  * The basic data type is instr_time, which all callers should treat as an
  * opaque typedef.  instr_time can store either an absolute time (of