update makefile and remove outdated benchs directory

[wxyucs]

the output after change:

Usage: make <target>

Targets:
help:                    ## Show the help.
##
## ================ development ================
debug:                   ## Build vsag with debug options.
test:                    ## Build and run unit tests.
asan:                    ## Build with AddressSanitizer option.
test_asan: asan          ## Run unit tests with AddressSanitizer option.
tsan:                    ## Build with ThreadSanitizer option.
test_tsan: tsan          ## Run unit tests with ThreadSanitizer option.
clean:                   ## Clear build/ directory.
##
## ================ integration ================
fmt:                     ## Format codes.
cov:                     ## Build unit tests with code coverage enabled.
test_parallel: debug     ## Run all tests parallel (used in CI).
test_asan_parallel: asan ## Run unit tests parallel with AddressSanitizer option.
test_tsan_parallel: tsan ## Run unit tests parallel with ThreadSanitizer option.
##
## ================ distribution ================
release:                 ## Build vsag with release options.
distribution:            ## Build vsag with distribution options.
libcxx:                  ## Build vsag using libc++.
pyvsag:                  ## Build pyvsag wheel.
clean-release:           ## Clear build-release/ directory.
install:                 ## Build and install the release version of vsag.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

see 7 files with indirect coverage changes

[jiaweizone]

LGTM

[inabao]

maybe we can set ENABLE_EXAMPLES=ON?

[wxyucs]

change the default value of ENABLE_EXAMPLES in another pull request

[LHT129]

LGTM