Revert "Reland "Define WEBRTC_ENABLE_SYMBOL_EXPORT if is_component_build=true.""

This reverts commit 93f7907973411ac3a5e5955b90f6a7f013195466.

Reason for revert: This CL was just needed in order to
have a WebRTC commit to pin in Chromium to test the
component build (this CL enables symbol exports).

Original change's description:
> Reland "Define WEBRTC_ENABLE_SYMBOL_EXPORT if is_component_build=true."
> 
> This is a reland of 03bc15c646d5b41d3169f2686316944788f640ed
> 
> I will revert this reland as soon as it lands because I just need
> to have a WebRTC commit to pin in Chromium in order to test the
> component build (this CL enables symbol exports).
> 
> Original change's description:
> > Define WEBRTC_ENABLE_SYMBOL_EXPORT if is_component_build=true.
> >
> > In order to land the component build support in Chromium, it is
> > easier to turn on symbols export every time that is_component_build=true
> > instead of setting rtc_enable_symbol_export=is_component_build in
> > Chromium (since is_component_build is not available in .gn).
> >
> > rtc_enable_symbol_export is still kept in the mix in order to turn
> > on symbol exports in any case a shared library will be added to the
> > WebRTC build.
> >
> > Bug: webrtc:9419
> > Change-Id: I5a7195826dea13d9a6f10a1160c35f2864bfa6c2
> > Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/157108
> > Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
> > Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
> > Cr-Commit-Position: refs/heads/master@{#29540}
> 
> No-Try: True
> TBR: kwiberg@webrtc.org
> Bug: webrtc:9419
> Change-Id: Ic5c3103da4ae5713cf5798b2eeaac396542d2026
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/159687
> Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
> Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#29771}

TBR=mbonadei@webrtc.org,kwiberg@webrtc.org

Change-Id: Ia1225d3619a90dfdd35c561da7940fe5bf9f7452
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:9419
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/159688
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29772}

BUILD.gn

@@ -111,7 +111,7 @@ config("common_inherited_config") {
   cflags = []
   ldflags = []
 
-  if (rtc_enable_symbol_export || is_component_build) {
+  if (rtc_enable_symbol_export) {
     defines = [ "WEBRTC_ENABLE_SYMBOL_EXPORT" ]
   }