Remove unsupported method encode_prepack() from flash attention tests (#12970)

Summary: Remove unsupported method encode_prepack() from flash attention
tests which was causing CI failure.

Reviewed By: mcr229

Differential Revision: D79128259

Author: leafs1

backends/vulkan/test/op_tests/sdpa_test.cpp

@@ -583,7 +583,6 @@ void test_vulkan_flash_attention(
   ValueRef staging_out = graph.set_output_tensor(r_out);
 
   graph.prepare();
-  graph.encode_prepack();
   graph.prepack();
 
   // Copy inputs and run
@@ -841,7 +840,6 @@ void test_reference_flash_attention(
   ValueRef staging_out = graph.set_output_tensor(r_out);
 
   graph.prepare();
-  graph.encode_prepack();
   graph.prepack();
 
   graph.copy_into_staging(r_q.staging, q.const_data_ptr(), q.numel());