Remove two useless attributes that snuck in while https://codereview.chromium.org/1221073004 was in review.

No intended behavior change.  Should fix these errors on the clang/win bots:

../..\components/proxy_config/proxy_prefs.h(18,6) :  error: 'dllexport' attribute only applies to variables, functions and classes [-Werror,-Wignored-attributes]
enum PROXY_CONFIG_EXPORT ProxyMode {
     ^
../..\components/proxy_config/proxy_config_export.h(12,40) :  note: expanded from macro 'PROXY_CONFIG_EXPORT'
                                       ^
In file included from ../../components/proxy_config/proxy_prefs.cc:5:
../..\components/proxy_config/proxy_prefs.h(41,6) :  error: 'dllexport' attribute only applies to variables, functions and classes [-Werror,-Wignored-attributes]
enum PROXY_CONFIG_EXPORT ConfigState {
     ^
../..\components/proxy_config/proxy_config_export.h(12,40) :  note: expanded from macro 'PROXY_CONFIG_EXPORT'
                                       ^

BUG=504695
TBR=stevenjb,benwells

Review URL: https://codereview.chromium.org/1218803005.

Cr-Commit-Position: refs/heads/master@{#337470}