Revert "Only assert defined vars conditionally"

This reverts commit 05d4429.

I am going to implement what I believe is a better, more robust solution in a
later commit (after adding a failing test to cover the case that this commit
addresses).

tribits/core/package_arch/TribitsPackageMacros.cmake

@@ -440,19 +440,13 @@ macro(tribits_disable_optional_dependency  upstreamPackageName  reasonStr)
       "  Instead, please call this from the package's base CMakeLists.txt file"
       " '${${PACKAGE_NAME}_SOURCE_DIR}/CMakeLists.txt'" )
   endif()
-  # Get the variable names that are going to be set
+  # Get the variable names that are going to be set assert they exist already
   set(packageEnableVarName ${PACKAGE_NAME}_ENABLE_${upstreamPackageName})
+  assert_defined(${packageEnableVarName})
   string(TOUPPER  ${upstreamPackageName}  upstreamPackageName_UC)
   set(havePackageUpstreamPackageMacroVarName
     HAVE_${PACKAGE_NAME_UC}_${upstreamPackageName_UC})
-  # Assert that the vars already exist (to make sure the package and dependency exist)
-  if (${PROJECT_NAME}_ASSERT_DEFINED_DEPENDENCIES  IN_LIST
-      ${PROJECT_NAME}_ASSERT_DEFINED_DEPENDENCIES_ERROR_VALUES_LIST
-    )
-    # We only assert if all packages have to exist, which is not true in a reduced source tree
-    assert_defined(${packageEnableVarName})
-    assert_defined(${havePackageUpstreamPackageMacroVarName})
-  endif()
+  assert_defined(${havePackageUpstreamPackageMacroVarName})
   # Set the variables to OFF in local and project-level scopes
   if (NOT "${reasonStr}" STREQUAL "")
     message("-- ${reasonStr}")