Remove IFX_SeekableStream::Flush().

It is pure virtual and it overrides IFX_SeekableWriteStream::Flush()
which is also pure virtual.

Change-Id: If48aaa8b5d6fe90c512781e0290cca5f7bcefc15
Reviewed-on: https://pdfium-review.googlesource.com/c/45551
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

core/fxcrt/fx_stream.h

diff --git a/core/fxcrt/fx_stream.h b/core/fxcrt/fx_stream.h
index 5d47378..32ef9a8 100644
--- a/core/fxcrt/fx_stream.h
+++ b/core/fxcrt/fx_stream.h
@@ -105,8 +105,6 @@
                   size_t size) override = 0;
   bool WriteBlock(const void* buffer, size_t size) override;
   bool WriteString(const ByteStringView& str) override;
-
-  bool Flush() override = 0;
 };
 
 #endif  // CORE_FXCRT_FX_STREAM_H_