remove outdated note on pthreads

src/README

@@ -251,26 +251,6 @@ For a build without `--prefix' (or with `--enable-origtree') and without
 `--enable-shared', you can safely move the install tree, because all
 file references within the installation are relative.
 
-OpenGL, Unix, and pthreads
---------------------------
-
-On some Unix systems, programs that use the OpenGL library must also
-link to pthreads.  To use GRacket's OpenGL support on these systems,
-GRacket must be configured with the `--enable-pthread' option.
-Unfortunately, Racket's normal stack handling and use of signals (for
-its own thread scheduling) do not interact well with pthreads.  Thus,
-when pthreads are enabled, Racket and GRacket cannot use interupt
-timers, and the stack is limited to 1MB.  These restrictions can degrade
-performance and thread-responsiveness.
-
-If `configure' detects that OpenGL requires -lpthread for linking, and
-if `--enable-pthread' is not specified, `configure' prints a warning and
-disables GL support.  On some systems, including at least FreeBSD with
-the MESA GL implementation (but not the NVIDIA implementation, which
-does not need pthreads), `configure' cannot detect the need for
-pthreads.  On these platforms, GL support will be included in GRacket,
-but it will not work properly unless `--enable-pthread' is specified.
-
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  CGC versus 3m
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