Remove obsolete target covtest

This target dates back from before basic-build-test.sh existed. Refer to
that script instead.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 3a870a7..6bf9fae 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -304,13 +304,13 @@
     # additional convenience targets for Unix only
     if(UNIX)
 
-        ADD_CUSTOM_TARGET(covtest
-            COMMAND make test
-            COMMAND programs/test/selftest
-            COMMAND tests/compat.sh
-            COMMAND tests/ssl-opt.sh
-        )
-
+        # For coverage testing:
+        # 1. Build with:
+        #         cmake -D CMAKE_BUILD_TYPE=Coverage /path/to/source && make
+        # 2. Run the relevant tests for the part of the code you're interested in.
+        #    For the reference coverage measurement, see
+        #    tests/scripts/basic-build-test.sh
+        # 3. Run scripts/lcov.sh to generate an HTML report.
         ADD_CUSTOM_TARGET(lcov
             COMMAND scripts/lcov.sh
         )