Remove IFX_SystemHandler.

This CL folds IFX_SystemHandler into CFX_SystemHandler. Methods which either
had no implementation, or returned a default value have been removed.

Review URL: https://codereview.chromium.org/1923093002
diff --git a/BUILD.gn b/BUILD.gn
index 4c974a9..747c630 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -57,6 +57,8 @@
 
 static_library("pdfium") {
   sources = [
+    "fpdfsdk/cfx_systemhandler.cpp",
+    "fpdfsdk/cfx_systemhandler.h",
     "fpdfsdk/fpdf_dataavail.cpp",
     "fpdfsdk/fpdf_ext.cpp",
     "fpdfsdk/fpdf_flatten.cpp",