cmake : remove precompiled headers (#28892)

This commit removes the precompiled headers that I added in Commit
3bcfeb700  ("cmake : add PCH and unity build to improve build times
(#28091)").

The motivation for this is that this looked good when developing this
but has caused multiple issues that I had taken into consideration and
we have decided to remove it and only keep the unity builds from the
above commit.

Refs: https://github.com/ggml-org/llama.cpp/pull/28882#issuecomment-5662272126

tools/server/CMakeLists.txt

@@ -1,13 +1,5 @@
 include_directories(${CMAKE_CURRENT_SOURCE_DIR} ${CMAKE_CURRENT_BINARY_DIR})
 
-# MSVC emits a PCH bookkeeping symbol that WINDOWS_EXPORT_ALL_SYMBOLS exports as an ambiguous "__"
-
-set(LLAMA_SERVER_PCH ON)
-
-if (BUILD_SHARED_LIBS AND CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
-    set(LLAMA_SERVER_PCH OFF)
-endif()
-
 # server-context containing the core server logic, used by llama-server and CLI
 
 set(TARGET server-context)
@@ -41,10 +33,6 @@ target_include_directories(${TARGET} PRIVATE ../mtmd)
 target_include_directories(${TARGET} PRIVATE ${CMAKE_SOURCE_DIR})
 target_link_libraries(${TARGET} PUBLIC llama-common mtmd ${CMAKE_THREAD_LIBS_INIT})
 
-if (LLAMA_SERVER_PCH)
-    target_precompile_headers(${TARGET} PRIVATE ${CMAKE_SOURCE_DIR}/common/common.h)
-endif()
-
 # llama-server-impl: server logic, reusable by app
 
 set(TARGET llama-server-impl)
@@ -62,10 +50,6 @@ target_include_directories(${TARGET} PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
 target_include_directories(${TARGET} PRIVATE ../mtmd ${CMAKE_SOURCE_DIR})
 target_link_libraries(${TARGET} PUBLIC server-context llama-ui cpp-httplib ${CMAKE_THREAD_LIBS_INIT})
 
-if (LLAMA_SERVER_PCH)
-    target_precompile_headers(${TARGET} PRIVATE ${CMAKE_SOURCE_DIR}/common/common.h)
-endif()
-
 add_dependencies(${TARGET} llama-ui-assets)
 
 if(LLAMA_TOOLS_INSTALL)