Autotools: remove commented out -Wdouble-promotion enabling

An initial attempt was made to enable this in 490481af, but this was subsequently commented out due to lacking compiler support. There has been an effort to incrementally re-enable the commented out warnings (e.g. f54fdcd8). However, in 2024 it no longer seems desirable to enable -Wdouble-promotion. Double-precision floating-point support is near-ubiquitous in hardware and there are numerous places where Graphviz code implicitly promotes float values to double. If any of these are shown to be a performance concern, we could revisit this, but for the most part Graphviz code assumes double is supported and never a relevant bottleneck for any of its operations.