[python-package] remove unnecessary Boost files from sdists

[jameslamb]

Contributes to #5061.

The OpenCL-based GPU builds of LightGBM rely on the Boost headers included in the repo

LightGBM/CMakeLists.txt

Line 179 in 36fe73a

set(BOOST_COMPUTE_HEADER_DIR ${PROJECT_SOURCE_DIR}/external_libs/compute/include)

but link against already-compiled Boost libraries, expected to be pre-installed.

LightGBM/CMakeLists.txt

Line 187 in 36fe73a

find_package(Boost 1.56.0 COMPONENTS filesystem system REQUIRED)

As a result, the only Boost files that need to be packaged in sdists are the headers in external_libs/compute/include. This PR proposes removing the other Boost files, to reduce the size of sdists and the time it takes to build them.

[github-actions]

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