build: remove -Wno-unused-result from gcc options (#5214)

Background: this flag was originally added in commit c5494ac
(2015-11-24) with the message "...now builds linux+gcc...". It was later
refactored in commit 62d7164 (2017-01-23). From investigation, it
seems to be preventative rather than reactive because it does not
actively suppress warnings, but `-Werror` is enabled under the condition
NOT MSVC + STOP_ON_WARNING (originally in CMakeLists.txt, now in
compiler.cmake:126).

Compile tests without `-Wno-unused-result` but with `-Wunused-result`
explicitly enabled were conducted on the following commits, and all
compiled successfully without unused result warnings. Also, tests
confirmed that the warning triggers if there is an unused return value.
- 312793a (2026-01-01)
- 682825f (2025-01-01)
- 48362b2 (2024-01-01)
- 3b8807d (2023-01-01)
- 62d7164 (2017-01-23): refactored
- c5494ac (2015-11-24): originally added

Decision: the flag can be safely removed. Removing it allows gcc to
catch unused return values, which is required for the `OIIO_NODISCARD`
and `OIIO_NODISCARD_ERROR` macros to work as intended.

Related: #5196
Assisted-by: Claude / Opus 4.7


Signed-off-by: Luna Kim <177369799+luna-y-kim@users.noreply.github.com>

Author: luna-y-kim

src/cmake/compiler.cmake

@@ -185,7 +185,6 @@ endif ()
 if (CMAKE_COMPILER_IS_GNUCC AND NOT (CMAKE_COMPILER_IS_CLANG OR CMAKE_COMPILER_IS_APPLECLANG))
     # gcc specific options
     add_compile_options ("-Wno-unused-local-typedefs")
-    add_compile_options ("-Wno-unused-result")
     add_compile_options ("-Wno-aligned-new")
     add_compile_options ("-Wno-noexcept-type")
 endif ()