Enable -Wunused-function in cmake builds for tests

This has been the case when building with make since
d3d8a64dfa7c1b006c854b45f47e10261cf0019a. Be consistent.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 45854f1..2431e40 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -140,10 +140,6 @@
 # on non-POSIX platforms.
 add_definitions("-D_POSIX_C_SOURCE=200809L")
 
-if(CMAKE_COMPILER_IS_GNUCC OR CMAKE_COMPILER_IS_CLANG)
-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wno-unused-function")
-endif(CMAKE_COMPILER_IS_GNUCC OR CMAKE_COMPILER_IS_CLANG)
-
 if(CMAKE_COMPILER_IS_CLANG)
     set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wdocumentation -Wno-documentation-deprecated-sync -Wunreachable-code")
 endif(CMAKE_COMPILER_IS_CLANG)