LibGfx: Remove unused BMPWriter::set_compression()

This has always been unused, and after #8440 BMPWriter::dump()
unconditionally writes to m_compression, meaning even if this
method was called, it would have no effect.

Userland/Libraries/LibGfx/BMPWriter.cpp

@@ -67,7 +67,7 @@ static ByteBuffer write_pixel_data(RefPtr<Bitmap const> bitmap, int pixel_row_da
     return buffer;
 }
 
-static ByteBuffer compress_pixel_data(ByteBuffer const& pixel_data, BMPWriter::Compression compression)
+ByteBuffer BMPWriter::compress_pixel_data(ByteBuffer const& pixel_data, BMPWriter::Compression compression)
 {
     switch (compression) {
     case BMPWriter::Compression::BI_BITFIELDS:

Userland/Libraries/LibGfx/BMPWriter.h

@@ -16,11 +16,6 @@ class BMPWriter {
 public:
     BMPWriter() = default;
 
-    enum class Compression : u32 {
-        BI_RGB = 0,
-        BI_BITFIELDS = 3,
-    };
-
     enum class DibHeader : u32 {
         Info = 40,
         V3 = 56,
@@ -29,10 +24,16 @@ public:
 
     ByteBuffer dump(RefPtr<Bitmap const>, DibHeader dib_header = DibHeader::V4);
 
-    inline void set_compression(Compression compression) { m_compression = compression; }
-
 private:
+    enum class Compression : u32 {
+        BI_RGB = 0,
+        BI_BITFIELDS = 3,
+    };
+
+    static ByteBuffer compress_pixel_data(ByteBuffer const&, Compression);
+
     Compression m_compression { Compression::BI_BITFIELDS };
+
     int m_bytes_per_pixel { 4 };
     bool m_include_alpha_channel { true };
 };