Revert "Define WEBRTC_ENABLE_SYMBOL_EXPORT if is_component_build=true."

This reverts commit 03bc15c646.

Reason for revert: Breaks Chromium's FYI win debug build
https://ci.chromium.org/p/chromium/builders/webrtc.fyi/WebRTC%20Chromium%20FYI%20Win%20Builder%20(dbg)/4341

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}

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

Change-Id: Ib6032c6f8bfb6a0b5eef559430cfae4eda3da0e0
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:9419
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/157426
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29544}

BUILD.gn

@@ -106,7 +106,7 @@ config("common_inherited_config") {
   cflags = []
   ldflags = []
 
-  if (rtc_enable_symbol_export || is_component_build) {
+  if (rtc_enable_symbol_export) {
     defines = [ "WEBRTC_ENABLE_SYMBOL_EXPORT" ]
   }