chore: remove dead code and unused config

[ozan956]

Cleanup around the dead code that I am aware so long but did not have a time to fix them all.

This commit removes:

• device/camera/esp32_ov2640_old.cpp (whole file, superseded)
• arch/arm/cm7_common.c and arch/arm/cm7_fft.c (comment-only stubs; functionality moved to dwt_timer.c / cmsis_fft.c)
• core/image.h: GMMStats typedef (never instantiated)
• imgproc/filter.h: SepFilter2DContext struct (never instantiated)
• imgproc/filter.c: commented-out dead wrapper() function, and unused STRONG/WEAK macros (Canny hardcodes the literals instead)
• imgproc/fft.c: multiply() function (not declared in fft.h, no callers)
• imgproc/segmentation.c: GMMComponent struct, GMM_COMPONENTS macro, and gaussian_prob() (none called by grabCutLite or anything else)
• device/serial/stm32_uart.c: UART_CMD_CAPTURE/UART_CMD_SEND macros (code uses local "STR"/"STW" arrays instead), and the tx_flag/rx_flag globals plus their writes in the HAL UART callbacks (write-only, never read; also removes an uninitialized-tx_flag bug since only rx_flag was zero-initialized)
• board/stm32f7/configs.h: LCD_FRAME_BUFFER and CAMERA_FRAME_BUFFER defines (never read; display code uses FRAME_BUFFER)
• embedDIP_configs.h: derived EMBED_DIP_HAS_CAMERA/EMBED_DIP_HAS_DISPLAY macros and ENABLE_UART_LOGGING defines (all board sections; never consumed anywhere)
• CMakeLists.txt: EMBEDDIP_BUILD_DOCS and EMBEDDIP_ENABLE_UART_LOGGING options, plus the associated ENABLE_UART_LOGGING compile definition (neither option was consumed elsewhere in the build)