Remove a useless attribute in a file only built with the gyp build.

Remove a useless attribute in a file only built with the gyp build.

This is a follow-up to https://codereview.chromium.org/1221073004 which was
probably authored using the gn build.

Should fix:
..\..\components/session_manager/core/session_manager.h(17,6) :  error: 'dllexport' attribute only applies to variables, functions and classes [-Werror,-Wignored-attributes]
enum SESSION_EXPORT SessionState {
     ^
..\..\components/session_manager/session_manager_export.h(12,35) :  note: expanded from macro 'SESSION_EXPORT'
                                  ^

https://codereview.chromium.org/981023002/#msg17 asks if it's intentional that
this file is built in the gyp build but not in the gn build.

No intended behavior change.

BUG=504695
TBR=benwells,nkostylev

Review URL: https://codereview.chromium.org/1220043007

Cr-Commit-Position: refs/heads/master@{#337537}