Remove SEASTAR_MODULE definition - not needed

SEASTAR_MODULE is not needed because all Seastar headers are included
in the module purview (after 'export module seastar;'). Everything declared
in the module purview is automatically part of the module's interface and
exported, including:
- All seastar namespace symbols
- std::hash specializations in namespace std

The SEASTAR_MODULE_EXPORT macros in headers now expand to nothing, but this
is fine - the declarations are still exported because they're in the module
purview. The explicit 'export' keywords were redundant.

This simplifies the build: no special compile definitions needed for module builds.

Co-authored-by: avikivity <[email protected]>

Author: Copilot

src/CMakeLists.txt

@@ -99,10 +99,6 @@ target_compile_definitions (seastar-module
     SEASTAR_SCHEDULING_GROUPS_COUNT=${Seastar_SCHEDULING_GROUPS_COUNT}
   PRIVATE
     ${Seastar_PRIVATE_COMPILE_DEFINITIONS})
-# Define SEASTAR_MODULE only for the module interface file, not implementation files
-set_source_files_properties (seastar.cppm
-  PROPERTIES
-    COMPILE_DEFINITIONS "SEASTAR_MODULE")
 target_compile_options (seastar-module
   PUBLIC
     -U_FORTIFY_SOURCE)