MDEV-8533 Debug embedded server does not build on Windows

pre/CMakeLists.txt defines CMAKE_DEBUG_POSTFIX which causes a different
library name on Windows debug build (pcred.lib rather than pre.lib)
However MERGE_LIBRARIES macro that is used to create static embedded
library (out of other static libraries), can  not handle per-configuration
library names. Thus the build fails with "pre.lib not found"

Fix is to remove unnecessary CMAKE_DEBUG_POSTFIX

pcre/CMakeLists.txt

@@ -311,11 +311,6 @@ IF(MSVC)
 ENDIF(MSVC)
 
 SET(CMAKE_INCLUDE_CURRENT_DIR 1)
-# needed to make sure to not link debug libs
-# against release libs and vice versa
-IF(WIN32)
-  SET(CMAKE_DEBUG_POSTFIX "d")
-ENDIF(WIN32)
 
 SET(targets)