doc: Remove obsolete hint for valgrind stack size

Also don't mention exhaustive_tests without explanation. They're
included in our test suite (`make check`) anyway.

README.md

@@ -66,18 +66,9 @@ libsecp256k1 is built using autotools:
     $ ./autogen.sh
     $ ./configure
     $ make
-    $ make check
+    $ make check  # run the test suite
     $ sudo make install  # optional
 
-Exhaustive tests
------------
-
-    $ ./exhaustive_tests
-
-With valgrind, you might need to increase the max stack size:
-
-    $ valgrind --max-stackframe=2500000 ./exhaustive_tests
-
 Test coverage
 -----------