Remove RTC_EXPORT from RTCStatsMember<T> template declaration.

Starting from [1], explicit template declaration/definition is in use
for this template so there is no need to RTC_EXPORT its declaration.
Doing so leads to this error on clang-cl:

../../third_party/webrtc\api/stats/rtc_stats.h(372,1): error: explicit instantiation declaration should not be 'dllexport' [-Werror,-Wdllexport-explicit-instantiation-decl]
WEBRTC_DECLARE_RTCSTATSMEMBER(bool);
^
../../third_party/webrtc\api/stats/rtc_stats.h(369,3): note: expanded from macro 'WEBRTC_DECLARE_RTCSTATSMEMBER'
  extern template class RTC_EXPORT_TEMPLATE_DECLARE(RTC_EXPORT)  \
  ^
../../third_party/webrtc\api/stats/rtc_stats.h(287,7): note: attribute is here
class RTC_EXPORT RTCStatsMember : public RTCStatsMemberInterface {
      ^
../..\third_party/webrtc/rtc_base/system/rtc_export.h(24,31): note: expanded from macro 'RTC_EXPORT'

Full log: https://ci.chromium.org/p/chromium/builders/try/win_chromium_compile_dbg_ng/430931

[1] - https://webrtc-review.googlesource.com/c/src/+/158795

Bug: webrtc:9419
Change-Id: I9f0893ae26b45049f186e19f862a1d138a320a24
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/158891
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29703}

api/stats/rtc_stats.h

@@ -284,7 +284,7 @@ class RTCStatsMemberInterface {
 // The supported types are the ones described by
 // |RTCStatsMemberInterface::Type|.
 template <typename T>
-class RTC_EXPORT RTCStatsMember : public RTCStatsMemberInterface {
+class RTCStatsMember : public RTCStatsMemberInterface {
  public:
   explicit RTCStatsMember(const char* name)
       : RTCStatsMemberInterface(name, /*is_defined=*/false), value_() {}