Don't restrict CMAKE_BUILD_TYPE

This prevents projects that add Eigen using `add_subdirectory` from using their own custom CMAKE_BUILD_TYPE and have Eigen respect the same custom flags.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 81c98a0..e4ca76c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -20,13 +20,6 @@
   set(CMAKE_BUILD_TYPE "Release")
 endif()
 
-string(TOLOWER "${CMAKE_BUILD_TYPE}" cmake_build_type_tolower)
-if(    NOT cmake_build_type_tolower STREQUAL "debug"
-   AND NOT cmake_build_type_tolower STREQUAL "release"
-   AND NOT cmake_build_type_tolower STREQUAL "relwithdebinfo")
-  message(FATAL_ERROR "Unknown build type \"${CMAKE_BUILD_TYPE}\". Allowed values are Debug, Release, RelWithDebInfo (case-insensitive).")
-endif()
-
 
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 # retrieve version information                                               #